1. Field of the Invention
This invention relates to an image sensing apparatus (inclusive of a digital still camera, a digital movie camera and a personal digital assistant having a camera function).
2. Description of the Related Art
In a digital still camera that uses a solid-state image sensing device such as a CCD, a shutter operation can be performed, without providing a mechanical shutter, by controlling sweep-out of unnecessary signal charge, which has been stored in the solid-state image sensing device, and read-out of signal charge. This is a so-called electronic shutter operation.
If the aperture of a photodiode is widened in order to improve the sensitivity of a solid-state image sensing device, this leads to storage of a large quantity of smear signal charge, which causes the occurrence of smear. (Signal charge that causes smearing shall be referred to as “smear signal charge”.) Further, if the number of pixels of the solid-state image sensing device is increased, the time needed to read signal charge out of the solid-state image sensing device lengthens, as a consequence of which even more smear signal charge accumulates in the device. For these reasons, some digital still cameras having an electronic shutter capability are provided with a mechanical shutter as well.
When exposure time of a solid-state image sensing device is controlled using a mechanical shutter, however, the following problems arise: First, since operating time such as that for opening and closing the mechanical shutter is required, the intervals at which successive frames can be shot continuously cannot be made as short as in the case of the electronic shutter. Second, since a mechanical shutter is operated, durability declines and more power is consumed.